1. Field of the Invention
The present invention relates to aplanatic reflector arrangements for offset multibeam ground station or satellite antennas and, more particularly, to multibeam antenna arrangements comprising N reflectors disposed in a particular sequential arrangement with two of the reflectors being slightly deformed in a predetermined manner to cause substantial elimination of the aberrations of astigmatism and coma over a wide area of the focal surface of the antenna.
2. Description of the Prior Art
Of considerable interest in practice is the problem of modifying an existing or a new antenna design so as to reduce or substantially eliminate aberrations which might be produced. More particularly, Cassegrainian and Gregorian reflector arrangements are needed for multibeam ground station and satellite antennas. In these antennas, an arrangement of two reflectors, a paraboloid and either a hyperboloid or an ellipsoid, is combined with several feeds disposed in the vicinity of a focal point. Each feed produces a beam whose direction is determined by the feed displacement from the focal point. This displacement has been found to normally cause aberrations due primarily to astigmatism and coma. Various arrangements have been derived to correct one or more of such aberrations in antennas. One such arrangement was for Spherical, Coma and Chromatic Aberrations" by A. R. Panicali et al in Proceedings of the IEEE, Vol. 59, No. 1, February, 1971, at pp. 311-312 where a corrugated reflector with varying depths of corrugations was suggested.
In the article "Astigmatic Correction by a Deformable Subreflector" by W-Y Wong et al in AP-S International Symposium, Vol. II, Seattle, Wash., 1979, at pp. 706-709, a mechanically deformable subreflector is suggested for providing a first order astigmatic correction. Other astigmatic correction arrangements have been disclosed in, for example, U.S. Pat. No. 4,145,695 issued to M. J. Gans on Mar. 20, 1979 and U.S. Pat. No. 4,224,626 issued to R. L. Sternberg on Sept 23, 1980. The Gans patent provides an astigmatic launcher reflector for each off-axis feedhorn which has a reflector having a curvature and orientation of its two orthogonal principal planes of curvature which are chosen in accordance with specific relationships. The Sternberg patent uses a lens having an elliptical periphery and surfaces defined by a system of nonlinear partial differential equations.
U.S. Pat. No. 4,166,276 issued to C. Dragone on Aug. 28, 1979 relates to an offset antenna having improved symmetry in the radiation pattern and comprising a curved focusing main reflector, at least two conic subreflectors and a feedhorn, the combination of these elements being oriented such that the feedhorn is disposed at the focal point of the combined confocal reflectors and in a manner to coincide with the equivalent axis of the antenna system. Such arrangement eliminates astigmatism to a first order approximation.
More recently, U.S. patent appln. Ser. No. 209,943 filed on Nov. 24, 1980 for T. Chu, now U.S. Pat. No. 4,339,757, and U.S. patent appln. Ser. No. 209,944 filed on Nov. 24, 1980 for E. A Ohm, now U.S. Pat. No. 4,343,004, where each disclose different astigmatic correction means comprising a first and a second doubly curved subreflector which are curved in orthogonal planes to permit the launching of an astigmatic beam of constant size and shape over a broadband range.
The foregoing astigmatic correction arrangements, however, are primarily designed to provide such correction in a very limited portion of the focal surface. The problem remaining in the prior art is to provide an antenna arrangement for multibeam transmission which will correct for astigmatism and also coma over a wide area of the focal surface of the antenna arrangement.